Known apparatuses for extracorporeal treatment of blood include at least one treatment unit (for example a dialyzer or a filter, or an ultra-filter or a plasma filter or a filter unit of any other nature) having a semipermeable membrane which separates the treatment unit into two chambers. An extracorporeal blood circuit allows the circulation of blood taken from a patient through the first chamber of the treatment unit. At the same time, and typically in a counter-current direction to the blood flow, a treatment fluid is circulated through a special circuit in the second chamber of the treatment unit. This type of equipment for blood treatment may be used for the removal of solutes and excess fluid from the blood of patients suffering from renal failure.
The extracorporeal blood circuit also includes an arterial and a venous chamber, also called bubble-traps, respectively located on a blood removal line from the patient and on a return blood line to the patient. The venous and arterial chambers, during the treatment, contain a predetermined quantity of blood so that the chambers are filled to certain levels (depths), and a predetermined quantity of gas (air) in the remaining part of the chamber. For a safe operation of the extracorporeal treatment, the level of blood should never fall below a critical minimum level that could lead to the introduction of air into the extracorporeal circulation lines and subsequent potential infusion of the air into the circulatory system of the patient, with serious consequences.
Since the risks of such an event exist, and the problems caused to the patient are extremely serious, if not critical, the known extracorporeal blood treatment machines are equipped with safety systems that may detect such an event and, should it occur, place the patient in safety. In particular, on the return blood line that returns blood back to the patient, upstream the vascular access and downstream of the venous chamber, a device is present which is directly connected to the control unit of the machine and configured for the detection of air bubbles in the blood. In the event of a bubble of air is detected in the venous line, the control unit activates a patient safety procedure for the “isolation” of the patient by at least closing clamps on the extracorporeal blood circuit and/or shutting down the blood pump.
In addition to this safety device, some machines are also equipped with appropriate blood level sensors in the venous chamber (more rarely also in the arterial chamber). Such blood level sensors signal when a minimum level is reached that requires intervention of specialized personnel to restore the correct quantity of blood in the chamber such as to avoid risks to the patient.
These systems, while fulfilling the tasks for which they are designed, incur additional costs and require changes to the hardware of a machine on which they are or are to be installed. Particularly because of the cost, these security systems are generally present only on the return line of the blood, downstream of the treatment unit.
Furthermore, it is also worth mentioning that bubble sensors are generally able to reliably detect only bubbles of a certain size, while they are substantially insensitive to micro-bubbles dissolved in blood. Although in the past dissolved micro-bubbles were not considered dangerous to patients, recent studies (e.g. “Microemboli, developed during hemodialysis, pass the lung barrier and may cause ischemic lesions in organs such as the brain” from journal “NDT Nephrology Dialysis Transplantation”, Volume 25, issue 8, pages 2691-2695, August 2010, by Ulf Forsberg, Per Jonsson, Christofer Stegmayr and Bernd Stegmayr) have linked some typical disorders of chronic patients—such as pulmonary hypertension and other ischemic problems—to the quantity of air, in the form of micro-bubbles, generated during dialysis and not detected by current safety systems.
It should be noted in this regard that generation of micro-bubbles mainly occurs because of the entry of air into the blood removal line, for example due to a low level of blood in the arterial blood chamber. A low level of blood may for instance be caused by insufficient machine priming or by an infusion. Bubbles may then get into the bloodstream and reach the dialyzer, which micronizes the air bubbles making them difficult to detect with conventional sensors.
In order to address the problem of determining and monitoring blood levels in blood chambers, U.S. Pat. No. 7,013,727 discloses a method for determining the blood level in a chamber used in dialysis machines which applies the ideal gas law. According to this patent, a change in the volume occupied by the gas in the chamber is linked to pressure and the level of blood in the chamber is detected. This methodology, while enabling blood level determination without using a level sensor, requires additional hardware (a further sensor) in addition to that already present on the machine.